Nematic liquid crystalline compositions are used in various electro-optical devices which involve the modulation of light, such as light valves and optical display devices. Such light valves typically are controlled by an electric field and operate when the liquid crystalline material is in its mesomorphic state. Generally, when no electric field is applied to a nematic liquid crystalline material, a device containing a thin layer of this material is relatively transparent to light. When an electric field is applied to the liquid crystalline material above a threshold value, the device appears to become opaque in the region of the field. This opacity is believed to be due to the scattering of light by domains of the liquid crystal molecules which align themselves in the field. A dynamic scattering effect is seen when the aligned molecules are perturbed by ions "bumping" into them and causing movement thereof.
The electro-optical effect due to alignment of domains of the nematic liquid crystal molecules in an electric field may be employed in transmissive, reflective, or absorptive-type flat-panel displays, in light shutters, and in other applications.
Nematic liquid crystalline materials exhibit a characteristic thread texture when properly viewed, and this texture is believed to be the boundaries of many domains or clusters in which the liquid crystalline molecules have a fixed orientation. According to the cluster theory of nematic liquid crystals, the clusters are randomly oriented, which accounts for the light scattering properties and for the turbid appearance of a fairly large volume. In addition, nematic liquid crystalline molecules typically are electrically and magnetically anisotropic. Thus, upon the application of an electric or magnetic field to a layer of randomly oriented nematic liquid crystals, the clusters tend to become oriented in a particular direction. This orientation results in changes in the light-scattering and birefringent properties of the layer. The degree of orientation attained by the molecules usually is dependent upon the magnitude of the applied field. Accordingly, the light-scattering properties and birefringent properties of a layer of nematic liquid crystalline material can be modulated with an electric or magnetic field.
Numerous examples of liquid crystalline materials have been described in the literature. However, to date, there is little certainty involved in predicting the exact properties of any hypothetical liquid crystalline compound. In view of this lack of predictability, the search continues for suitable nematic liquid crystalline compounds. From the standpoint of ease of handling, it is desirable to have materials which have low crystal to mesomorphic transition temperatures and preferably the liquid crystalline materials should have a broad temperature range over which the nematic phase exists.